1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a polishing apparatus having an interlock function which can perform a predetermined operation such as shutdown of a plurality of components in the overall apparatus when an abnormality is detected in at least one of motors and actuators of the apparatus by, for example, measuring magnitudes of the torques of the motors or measuring magnitude of vibration of a top ring.
2. Description of the Prior Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical polishing.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished to a flat mirror finish while the top ring and the turntable are rotated.
In the conventional polishing apparatus, a motor for rotating the turntable, the motor for swinging a top ring, and a motor for rotating the top ring have such respective functions that the respective motors are stopped when an overcurrent occurs in the respective motors. In this case, the allowable overcurrent depends on the capacity of the respective motors.
However, in the method in which each of the motors is stopped by detecting an overcurrent of the respective motors, since the turntable and the top ring are rotated by the respective motors, even if one of the motors is stopped due to an overcurrent, the other motors continues to be rotated. In other words, even if one of the turntable and the top ring is stopped, the other continues to be rotated, and thus the stopping one is dragged by the other element which continues to be rotated. As a result, the semiconductor wafer held by the top ring is damaged, or the top ring or the associated component in a polishing unit is damaged.